If you're using a mobile phone for your custom design there is a thin black line at the bottom of the custom interface that says "ADD TO CART" that sometimes blends into the black edge of your phone.
If you're on a computer there should be a button to the right of your customization panel - try expanding your browser window if you don't see it.
Any other questions or if you need further assistance please email us at service@dynomighty.com and someone will respond in 24-48 hours.